1. Field of the Invention
The present invention relates to a Faraday rotator, which constitutes an optical isolator or an optical circulator for use in an optical fiber communication system, an optical recording system, an optical measurement system or the like.
2. Description of the Related Art
In an optical fiber communications system having a semiconductor laser as a light source, in particular, an optical system based on a high speed digital transmission or an analog direct modulation mode, if reflected light from optical connector joints, optical circuit components and the like which are used in an optical fiber circuit returns to the semiconductor laser or an optical amplifier, it becomes difficult to maintain high quality transmission due to degradation of frequency characteristics or generation of noises. An optical isolator is used for the purpose of removing the reflected light.
As shown in FIG. 11, a conventional optical isolator is constituted by a polarizer 6 and an analyzer 5 both of which transmit only light having a specific plane of polarization, a Faraday rotator 4P composed of a light transmissible substrate 1P and a laminate film 3P formed thereon and adapted to rotate the plane of polarization of light by 45 degrees, and a permanent magnet (not shown) for applying a magnetic field to the Faraday rotator. Among the constituent members of the optical isolator, a Faraday rotator has a primary influence on the performance of the optical isolator. It is important for the Faraday rotator to have a small element length required for rotating the plane of polarization by 45 degrees and a large light transmittance.
Up to now, the Faraday rotator has been made of a yttrium iron garnet (YIG) bulk single crystal (about 2 mm in thickness), or of a bismuth-substituted rare earth iron garnet (BiYIG) thick film single crystal (several hundred xcexcm in thickness) in which part of yttrium is substituted with bismuth having a large magneto-optical performance index. Recently, the BiYIG thick film single crystal, which is advantageous in downsizing the optical isolator, is employed in many cases.
Further, in recent years, the magneto-optical component (Faraday rotator) made of one-dimensional magneto-photonic crystal which causes the enhancement of the magneto-optical effect due to the localization of light is proposed. Though the above-mentioned magneto-optical component is of polycrystal with a thickness of several xcexcm, a large Faraday rotation angle can be obtained. Thus, a significant reduction is expected in size and cost of the optical isolator.
This one-dimensional magneto-photonic crystal is structured such that at least one kind of dielectric thin film and a magneto-optical thin film are laminated to form a Fabry-Perot resonator structural. Particularly, with such a structure that a magneto-optical tin film is sandwiched between two periodic dielectric multilayer films, in each of which two kids of dielectric thin fume having different refractive indexes from each other are laminated, magneto-optical effect can be greatly enhanced while it is manufactured easily.
However, such a simple structure causes a problem that light transmittance decreases when the number of layers of the periodic dielectric multilayer films is increased to obtain a larger Faraday rotation angle. It is shown that a multiple Fabry-Perot resonator in which a plurality of Fabry-Perot resonators stacked sandwiching a dielectric thin film is effective in solving the problem. However, this increases significantly the number of layers, pushing up manufacturing cost
Under the above-mentioned circumstances, the present inventors have proposed a Faraday rotator 4R with a reflection structure as shown in FIG. 12, in which a metal reflective film 2 is formed on a substrate 1, then a first periodic dielectric multilayer film 3r, a magneto-optical thin film 3m and a second periodic dielectric multilayer film 3rxe2x80x2, which has the same number of layers as the first periodic dielectric multiplayer film 3r are sequentially formed on the metal reflective film 2, and in which light Pi made incident on a side on which the metal reflective film 2 is not formed is reflected at the metal reflective film 2. In the Faraday rotator 4R with the above-mentioned structure, aluminum is used for the metal reflective film 2. Thus, the number of layers can be reduced substantially by half in comparison with a conventional transmission type Faraday rotator.
With the above-mentioned reflection structure, the Faraday rotator can be composed of a small number of layers. However, a film forming apparatus dedicated to forming the metal reflective film is required, which hinders reduction in manufacturing cost.
The present invention has been made to solve the above problem, and it is therefore an object of the present invention to provide a Faraday rotator with a reflection structure, which does not use a metal reflective film, that is, which is constituted of a magneto-optical thin film and dielectric multilayer films only.
In order to attain the above-mentioned object, according to a first aspect of the present invention, there is provided a Faraday rotator which includes a laminate film comprising a fist periodic dielectric multilayer film in which two kinds of dielectric thin films having refractive indexes different from each other are alternately laminated with regularity in each thickness thereof, a second periodic dielectric multilayer film in which two kinds of dielectric thin films having refractive indexes different from each other are alternately laminated with regularity in each thickness thereof, and a magneto-optical thin film which is sandwiched between the first and second periodic dielectric multilayer films, wherein the number of layers of the first periodic dielectric multilayer film is different from that of the second periodic dielectric multilayer film.
According to a second aspect of the present invention, there is provided a Faraday rotator including a laminate film comprising a first periodic dielectric multilayer film in which two kinds of dielectric thin films having refractive indexes different from each other are alternately laminated with regularity in each thickness thereof a second periodic dielectric multilayer film in which two kinds of dielectric thin films having refractive indexes different from each other are alternately laminated with regularity in each thickness thereof, and a magneto-optical thin film which is sandwiched between the first and second periodic dielectric multilayer films, wherein the first and second periodic dielectric multilayer films are each structured such that one dielectric thin film having a higher refractive index of the two kinds is in contact with the magneto-optical thin film, the number of layers of the first periodic dielectric multilayer film is larger than that of the second periodic dielectric multilayer film, and a dielectric thin film with a large refractive index is added between the laminate film and a substrate.
According to a third aspect of the present invention, in the second aspect an optical length of the additional dielectric thin film with a high refractive index is xcex/4 where xcex is a wavelength of light,
According to a fourth aspect of the present invention, in any one of the first to third aspects, at least one of layers constituting the periodic dielectric multilayer film is substituted by a magneto-optical thin film.